Error: No module named ' .... '
Blender cannot find your module because blender is searching for modules in the base path and your module is not there, so you need to append your modules path to
a path that blender can find it, so you need to execute some functions to do this.

import bpy
import sys
import os

# we get blend file path
filepath = bpy.data.filepath

# we get the directory relative to the blend file path
dir = os.path.dirname(filepath)

# we append our path to blender modules path
# we use if not statement to do this one time only
if not dir in sys.path:
   sys.path.append(dir )

# now blender is able to find our modules
import module_test_hello

# in case we edited our script file after blender had started 
# we reload the module via imp module
import imp
imp.reload(module_test_hello)

# we call function from the module now
module_test_hello.hello()